This invention relates to fault detection particulary, low level DC fault currents. In particular, it relates to an apparatus and a method for detecting ground faults in normally ungrounded multi-feeder DC distribution systems having significant capacitive reactance components and under the influence of strong electromagnetic fields. These situations are normally associated with utility power generation and distribution, industrial plants, and computer/electronic systems therein. In such systems, ground faults must be located without taking unaffected equipment out of service.
Generating stations and substations use 110 to 240 volt ungrounded battery systems to operate control systems and other DC devices. Some of the control systems are critical to plant operational integrity and must operate at all times. If a ground fault on the ungrounded battery system, if not isolated, will leak battery power to ground. This leakage may be sufficient to affect the battery system's operational integrity by lowering battery voltage. If two ground faults on opposite polarities of the same battery system occur simultaneously in the system, the battery may be shorted through ground. If two or more simultaneous ground faults on the same conductor occur, an undesirable bypassing of controlling devices may occur and cause malfunction or misoperation, consequently isolation and repair of the first fault must, therefore, be performed as quickly and efficiently as possible to minimize the chances that the whole battery system will be shorted or become inoperative.
The major components of an ungrounded DC distribution system usually include the DC battery assembly and battery charger. Main source conductors connect the battery assembly to the circuit breaker of a multi-feeder distribution panel, and the individual loads to those feeders. The type of loads associated with this system are motors, solenoids, relays, electronic monitoring equipment, and electronic control devices. A common characteristic associated with this type of system is, firstly, stray capacitance created by the distribution lines respect to ground and, secondly, input capacitive reactance of the loads. The value of the stray capacitance ranges from a few picofarads to 200 microfarads or more. This is an important characteristic since it plays an important part in the type of test equipment that can be used to locate ground fault currents.
A basic problem in such systems is the need to identify low level DC fault currents, namely, low to high impedance ground fault currents in the presence of much larger DC load currents and electromagnetically induced noise currents.
In DC ungrounded power distribution systems, it is important to determine whether a fault resistance exists between ground and any of the distribution lines or loads attached to those lines. Should a fault occur and the resistance value of the fault is below the predetermined alarm value it is important to locate the fault and remove it without interrupting service to the branch or feeder.
One methed used to located ground faults is to open circuit breakers one at a time until the fault disappears. The fault is then isolated during the time the branch circuit is de-energized and repaired. Should a ground fault occur on a critical branch circuit which cannot be opened for ground fault tracing, this method cannot be used.
A known ground detection circuit consists of a center-tapped high resistance connected across the DC source and an indicating voltmeter between the center tap and ground fault antwhere on the Dc system causes an indication of the voltmeter. Since the high resistance limits the ground fault current to a few milliamperes, the faulted equipment will not be tripped off when a low level fault occurs.
Other detection circuits consists of two resistors of equal value connected from each side of the main conductors to ground and a monitor instrument that can be switched between ground and the distribution lines. The monitoring instrument indicates a voltage imbalance when a ground fault exists between line and ground. The imbalance voltage represents a percentage of ground fault to be determined. This circuitry is succeptible to changing loads connected to the distribution lines and the influences of electromagnetically induced noise to identify low level fault currents effectively.
In other arrangements, the resistors are replaced by relays or solenoids driven by parallel windings. Each winding is connected between ground and one of the lines. When a ground fault condition exists an imbalance potential is created on one of the windings which causes current to flow through the windings to activate the electromechanical system and initiate a ground fault condition on the system. The limitation with this type of design is that the instrument is detecting a relatively high level fault current condition only but is unable to determine where the fault is located. Additional troubleshooting is needed to determine the location, and may require the injection of an AC signal into the DC system in order to trace the source of the fault. This method cannot be used on systems having large stray capacitance or sensitive electronic equipment as loads since the AC injected signal has to overcome very low impedance paths to ground. The lower the impedance the larger the AC injected signal needs to be to locate the fault. With high energy levels it is possible inadvertently to trip control devices or damage electronic equipment or loads connected to the system. The critical nature of these circuits requires them not to be turned off to locate the fault. Thus, a fault detection system is needed to locate the faulty equipment without interrupting these critical circuits.
It is also known elsewhere to test for DC faults in small systems employing grounded 12-volt battery type power supplies in automobiles and the like. Such grounded DC systems require the connection of an injector across terminals of the battery supply. Thereafter, a detector is applied over the wiring system with sound detection means so that an increasing sound would indicate where a DC fault exists. The limitations of this type of design has been identified above. Such systems operate in response to high DC fault currents in an environment where there is no significant capacitive or inductive reactances of consequence and where the DC system is effectively shut off when the fault detection is being made.
It is also known in AC systems to detect ground leakage by a relay which interrups the system so as to introduce a fault current in the sense of a pulsating input. Such systems, however, are of a nature that a D'Arsonval type meter of permanent magnet moving coil meter are used for detection of the pulsating input. Such a meter requires a current transformer suitable for detecting relatively large AC fault currents. This is unsuitable for measuring pulsating DC fault currents of a lower value. These detection systems are particularly unsuitable in high electrostatic and electromagnetic environments.
In another method of ground fault detection, a slope detector is used to detect an interrupt signal having a frequency of 2Hz per second. This signal is obtained by connecting two 5,000 ohm resistors from each side of the DC distribution line through an interrupter relay to ground. By controlling the opening and closing of the relay, the fault current is interrupted to generate a DC fault pulse. At the same time, a magnetic sensor and associated electronics is used to detect the rise and fall, namely, slope, of the interrupted DC signal. When a positive identification of the fault current is achieved, a periodic audio signal is generated or a flashing LED display is activated. With this detection method should the stray capacitance of the DC distribution system be above about 50 microfarads and the fault reisistance is above 5,000 ohm, the identification of the fault location may be difficult since the stray capacitance on the line can absorb most of the initial current generated by the interrupt pulse. This can cause a false slope signal to be produced and the detector circuit will acknowledge this as a fault condition. Also external electromagnetic interferences can produce an unwanted output signal that can interfere with the detector.
There is thus a need to overcome disadvantages of the prior system, and provide an effective means for detecting and locating faults in a supply system.